During the stretch blowing of plastic bottles, in particular PET bottles, it is desirable for reasons of quality assurance and process control to deter mine the mass or the mass proportion of individual regions of the bottles, such as for example the bottle bottom or the bottle shoulder. As is known, for this purpose bottles can be ejected from the production flow on a random sample basis and cut up in order to then weigh the sections to be inspected. However, it would be desirable to determine the mass of individual wall regions non-destructively and with less delay in the on-going product flow.
For this type of examination of the bottle bottom it is known from DE 10 2005 044 206 A2 that—in a flow of bottles to be inspected running continuously through an inspection device in neck handling—in each case light can be passed through the bottle bottom from below and, through the mouth of the bottle, the bottle bottom can be displayed in an image. According to the surface curvature of the bottle bottom, the camera image reveals here characteristic bright/dark structures, from which various quality features, in particular though the mass of the bottle bottom, can be determined non-destructively in the on-going process.
However, also the shoulder or neck region of the bottle is of particular interest, because when too little material is used in this region, the nitrogen and optionally carbon dioxide existing above the charged product can escape through the bottle wall to an unwanted extent and to the detriment of the product quality.
Indeed, it is known from WO 2008/027569 A2 that light can be passed approximately radially and aligned through the side wall of a rotationally symmetrical plastic bottle using a vertical light-emitting diode array and that the light transmission can be determined, locally dispersed, after light penetration through the bottle wall on both sides, using an appropriate number of detectors arranged one above the other. However, light is only passed through the bottle wall at points, so that the wall regions in the intervening spaces of the irradiating grid are not included. It may be however that it is just those wall sections that are not included which are particularly problematical. The irradiation grid also results in that the limits of the measurement range cannot be matched as required to the bottle shape. In addition an absolute value for the associated wall thickness must be determined from the relevant transmission through the bottle wall and in turn from this the mass of the associated wall section. This is however subject to comparatively large errors.